1. Field of the Invention
The present invention relates to communication systems utilizing frequency shift keyed (FSK) data transmission. In particular, the present invention relates to an FSK data transmission system in which an FSK signal is superimposed on a power line carrier. PG,3
2. Description of the Prior Art
In the past, there have been numerous proposals for communications systems which utilize existing power lines to carry the communication signals. One application of this type of communication system is for remote control of appliances and other electrical loads in homes and commercial buildings for convenience and energy management.
The power line carrier systems normally use a radio frequency (RF) signal which is superimposed with the power line carrier frequency (typically 60 Hz in the United States). The particular data transmission format for the RF signal can take one of several well-known formats.
One advantageous digital data transmission format is frequency shift keyed (FSK) transmission. An FSK transmitter modulates a reference frequency signal based upon the data to be transmitted, so that the transmitted signal has a frequency (f.sub.FSK) which is either slightly greater than or slightly less than the frequency (f.sub.O) of the reference signal. In typical FSK transmission systems, a frequency f.sub.FSK which is greater than f.sub.O represents a digital "1" while a frequency f.sub.FSK which is less than f.sub.O represents a digital "0". An FSK receiver receives and demodulates the transmitted FSK signal to produce a serial data stream at a predetermined data rate (or "baud rate"). Each bit of the data stream is based upon the frequency of the FSK signal during one bit time period.
In the prior art, FSK receivers have often been quite complicated and required extensive high precision components. The cost and complexity of FSK transmission systems has, as a result, limited the applicability of FSK transmitters and receivers in power line carrier communication systems.
There is a continued need for power line carrier FSK data systems which eliminate the need for costly precision components and yet provide reliable transmission of data.